Unorthodox mandate missing a bicycle (6)
I believe the answer is:
tandem
'bicycle' is the definition.
(I've seen this before)
'unorthodox mandate missing a' is the wordplay.
'unorthodox' is an anagram indicator.
'missing' suggests deleting specific letters.
'mandate' with 'a' removed is 'mndate'.
'mndate' anagrammed gives 'TANDEM'.
(Other definitions for tandem that I've seen before include "Two-seater bicycle" , "Two wheels for two" , "Two-person bike" , "Two-seater bike" , "Bicycle built for two" .)